1. Field of the Invention
The present invention relates to compositions and methods for reducing the permeabilities of subterranean zones, also known as conformance control, and, more particularly, to improved water-soluble polymeric compositions which form cross-linked gels in the zones.
2. Discussion of Related Art
When wells penetrating oil and gas producing subterranean formations are produced, water often accompanies the oil and gas. The water can be the result of a water producing zone communicated with the oil and gas producing formation by fractures, high permeability streaks and the like, or it can be caused by a variety of other occurrences which are well known to those skilled in the art such as water coning, water cresting, bottom water, channeling at the well bore, etc. In addition, secondary recovery techniques such as water flooding to stimulate production of oil involve injection of water under pressure at a distance from a production well to squeeze the oil out. However, in both cases the water moves in the formation along least hindered paths, so that the recovery technique may be inefficient, and, in the direct recovery, increased proportions of water are produced.
In enhanced recovery techniques such as water flooding, an aqueous flood or displacement fluid is injected under pressure into an oil containing subterranean formation by way of one or more injection wells. The flow of the aqueous fluid through the formation displaces oil contained therein and drives it to one or more producing wells. However, the aqueous displacement fluid often flows through the most permeable zones in the subterranean formation whereby less permeable zones containing oil are bypassed. This uneven flow of the aqueous displacement fluid through the formation reduces the overall yield of hydrocarbons from the formation.
Heretofore, enhanced recovery problems in a subterranean oil containing formation caused by permeability variations therein have been corrected by reducing the permeability of the subterranean formation flow paths having high permeability and low oil content. As a result, the subsequently injected aqueous displacement fluid is forced through flow paths having low permeability and high oil content. The techniques utilized to accomplish this high flow path permeability reduction, referred to in the art as “conformance control techniques,” have included injecting aqueous solutions of polymers and gelling agents into the high permeability flow paths whereby the polymers are gelled and cross-linked therein.
For example, water-soluble polymers including copolymers of acrylamide and acrylic acid cross-linked with chromium or other transition metal ions have been utilized heretofore. In accordance with an early technique, an aqueous solution of one or more of the polymers or copolymers mixed with a cross-linking metal ion is injected into the subterranean formation and allowed to cross-link therein. However, it has heretofore been found that the cross-linked gels formed have often been ineffective at high temperatures, i.e., at temperatures above about 80 EC because of the instability of the cross-linker or polymer. This has resulted in uncontrolled cross-linking rates (too rapid), cross-linker precipitation, polymer degradation, or an inefficient solution propagation. In attempts to correct these problems, the cross-linking metal ion has been coordinated with a ligand such as acetate or propionate to slow the reaction of the metal ion with the polymer. While this and other techniques have been utilized successfully, the use of some metal ions, e.g., chromium, has adverse environmental effects, and the metal ion used can be adsorbed by formation materials whereby it is prevented from functioning to cross-link the polymer.
U.S. Pat. No. 4,773,481 to Allison et al. issued on Sep. 27, 1988 describes a process for reducing the permeability of a subterranean formation by the cross-linking of water-soluble polymers of polyalkyleneimines and polyalkylenepolyamines with certain polymers which are anionic or hydrolyzable to form anionic polymers. Examples of the anionic polymers are polyacrylamide and alkylpolyacrylamides, copolymers of polyacrylamide and alkylpolyacrylamides with ethylene, propylene and styrene, polymaleic anhydride and polymethylacrylate, and hydrolysis products thereof. As described in the patent, when the water-soluble polymer and the anionic polymer are mixed, a viscous gel is quickly formed. In use, a solution of the water-soluble polymer is pumped into the subterranean formation first, followed by water to displace the water-soluble polymer from the wellbore to thereby prevent premature gelling upon introduction of the anionic polymer. Thereafter, the anionic polymer is pumped into the formation. This three-step procedure has a number of disadvantages in practice and is costly to perform, but it is necessary because the water-soluble polyalkylene imine or polyalkylenepolyamine reacts very quickly with the anionic polymer and cannot be premixed without premature gelation.
U.S. Pat. No. 6,192,986 to Phillip Lance Urlwin-Smith issued on Feb. 27, 2001 and assigned of record to Halliburton Energy Services, Inc., the specification of which is incorporated herein by reference in its entirety, describes a way of avoiding the use of metal ion cross-linking agents and of controlling the gelling rate of polymers whereby premixes of polymer and a gelling agent can be made and safely injected into a downhole formation without serious risk of premature gelation. The composition comprises a water-soluble copolymer comprising (i) at least one non-acidic ethylenically unsaturated polar monomer and (ii) at least one copolymerisable ethylenically unsaturated ester; and (iii) at least one organic gelling agent, characterized in that the gelling agent is a polyalkyleneimine, polyfunctional aliphatic amine, an aralkylamine, or a heteroaralkylamine. The gelling agents are free from metal ions, and are preferably water-soluble polymers capable of cross-linking the copolymers. Among the preferred water-soluble polymers for use as gelling agents are polyalkyleneimines, polyalkylenepolyamines, and mixtures thereof. Additional details concerning these polymers and their preparation are disclosed in U.S. Pat. No. 3,491,049. The preferred polyalkylenepolyamines are the polymeric condensates of lower molecular weight polyalkylenepolyamines and a vicinal dihaloalkane. The polyalkyleneimines are best illustrated by polymerized ethylene imines or propylene imine. The polyalkylenepolyamines are exemplified by polyethylene and polypropylenepolyamines. Other gelling agents which can be used include water-soluble polyfunctional aliphatic amines, aralkylamines, and heteroaralkylamines optionally containing other hetero atoms. The method of conformance control of a subterranean reservoir comprises: (a) injecting into a formation an aqueous solution of a composition of the invention; (b) allowing the solution to flow through at least one permeable zone in said formation; and (c) allowing the composition to gel. It is generally unnecessary to have any pre-cool step, especially in wells with bottom hole temperatures up to about 120° C. As the solution is pumped downhole and permeates into the zone, it heats up and eventually reaches the downhole temperature after which gelling occurs.
U.S. Pat. No. 6,196,317 to Mary Anne Hardy issued on Mar. 6, 2001 and assigned of record to Halliburton Energy Services, Inc., the specification of which is incorporated herein by reference in its entirety, describes the steps of introducing an aqueous solution of a chelated organic gelling agent and a copolymer of an ethylenically unsaturated polar monomer and an ethylenically unsaturated ester into a subterranean zone, and then allowing the aqueous solution to form a cross-linked gel in the zone. The chelated organic gelling agent is comprised of a water-soluble polyalkylene imine chelated with a metal ion, preferably polyethylene imine chelated with zirconium. The ethylenically unsaturated polar monomer in the copolymer is an amide of an unsaturated carboxylic acid, preferably acrylamide, and the ethylenically unsaturated ester in the copolymer is formed of a hydroxyl compound and an ethylenically unsaturated carboxylic acid such as acrylic acid, methacrylic acid and the like. A preferred unsaturated ester is t-butyl acrylate. In a further aspect, instead of utilizing the above-described copolymer which is rapidly cross-linked by the chelated gelling agent once the chelated gelling agent disassociates, the copolymer can be stabilized whereby it does not cross-link as rapidly at high temperatures and also has greater long-term gel strength after being cross-linked by forming it into a terpolymer or a tetrapolymer. That is, instead of a copolymer, the above-described ethylenically unsaturated polar monomer, preferably acrylamide, and the ethylenically unsaturated ester, preferably t-butyl acrylate, are reacted with AMPS® (2-acrylamido-2-methylpropane sulfonic acid) and/or N-vinylpyrrolidone to produce a terpolymer, e.g., polyacrylamide/t-butyl acrylate/AMPS® or polyacrylamide/t-butyl acrylate/N-vinylpyrrolidone or a tetrapolymer, e.g., polyacrylamide/t-butyl acrylate/AMPS®/N-vinylpyrrolidone. The most preferred terpolymer is polyacrylamide/t-butyl acrylate/N-vinylpyrrolidone. The compositions for reducing the permeability of a subterranean zone are basically comprised of water, a copolymer of an ethylenically unsaturated polar monomer, and an ethylenically unsaturated ester or a terpolymer or tetrapolymer of the aforesaid polar monomer and ester with AMPS® and/or N-vinylpyrrolidone, and a chelated organic gelling agent.
Although the above-described water-based polymer systems cross-linked with organic cross-linkers have some thermal stability at higher temperatures, further stability improvement is desirable. The maximum pumping time of those systems, when used as matrix sealants in conformance applications, is limited by the short cross-link time at formation temperature. This makes application of such systems in wells with a higher BHST or low injectivity in many cases unfeasible due to the large cooldown volumes required. At higher temperatures, the adjustment of pH and other known methods to delay the cross-link time at lower temperatures do not show any effect.
Another limitation on the use of the existing compositions and methods is the density of the fluids used. For example, the commonly used calcium chloride and calcium bromide brines are not feasible for mixing with such water-based polymer systems because they precipitate out the polymer.
There are continuing needs, however, for improved compositions and methods for reducing the permeabilities of subterranean zones using water-soluble polymeric components whereby the cross-linking of the components is effectively and simply controlled at high temperatures. There is also a continuing need for improved compositions and methods that enable higher density fluid mixtures than heretofore could be achieved.